The present invention relates to an electric motor such as a brushless DC motor, and an electric power steering apparatus employing this electric motor as a drive source.
As a power steering apparatus for easing steering effort of the driver, an electric power steering apparatus using power of an electric motor has been widely adopted for various vehicles in recent years in place of the conventional hydraulic power steering apparatus. In general, an electric power steering apparatus assists the steering operation of the driver by way of reducing the rotation speed of the electric motor and increasing the rotation torque instead by the reduction gear mechanism and then transmitting the rotation torque to the rack shaft through the rack and pinion mechanism.
The conventional electric power steering apparatus has a drawback that the rotation shaft (output shaft) of the electric motor as a drive source displaces in the axial direction. In order to prevent the axial displacement of the rotation shaft of the electric motor while rotatably supporting the same, for example, Japanese Laid-open Patent Application No. 2002-284023 (paragraph [0019]; FIG. 1) discloses a configuration, in which a bearing supports only one end of the rotation shaft of the electric motor such that the rotation shaft does not displace in the axial direction.
However, once the electric motor of the electric power steering apparatus is driven, due to unbalance of magnetic force or unbalance of the rotation shaft per se, a greater force (centrifugal force) is generated in the radial direction of the rotation shaft during the high speed rotation of the rotation shaft. For this reason, according to the configuration of Japanese Laid-open Patent Application No. 2002-284023, in which only one end of the rotation shaft of the electric motor is stationarily supported by the bearing so as not to displace in the axial direction, vibration is generated at the other end of the rotation shaft due to the greater force (centrifugal force) acting in the radial direction resulting from the rotation of the rotation shaft.
When vibration is generated on the rotation shaft during the actuation of the electric motor, the vibration is transmitted from the reduction gear mechanism joined to the rotation shaft through the steering shaft and to the steering wheel. Steering feeling upon maneuvering the steering wheel therefore deteriorates. Further, vibration generated on the rotation shaft is transmitted to the gear housing, by which the reduction gear mechanism and the like are accommodated and which is connected to the motor housing, and noise occurs from the gear housing.
In view of the above, the present invention seeks to provide an electric motor and an electric power steering apparatus employing this electric motor, which can reliably prevent a generation of vibration on the rotation shaft due to the radial force (centrifugal force) caused by the rotation of the rotation shaft.